Probing the long-term thrombotic safety of mRNA COVID-19 vaccines by image-based single-cell monitoring of platelet activity

A global concern has been raised about extremely rare, but serious cases of a thrombotic disorder, known as COVID-19 vaccine-induced immune thrombotic thrombocytopenia (VITT), associated with the Oxford-AstraZeneca and Johnson & Johnson / Janssen COVID-19 vaccines. While the pathogenesis of VITT remains not fully understood, a currently accepted theory is that the adenoviral vector vaccines produce antibodies that recognize platelet factor 4 (PF4), leading to platelet activation and aggregation, followed by thromboembolic complications. Therefore, the recommended methods of evaluation for VITT include complete blood count, anti-PF4-heparin antibody enzyme-linked immunosorbent assay, coagulation tests, and medical imaging. However, none of these methods can directly probe platelet activity (i.e., platelet activation and aggregation) and study potential non-PF4 factors for increased platelet activity. Here we report a method for directly probing vaccine-induced platelet activity and hence assessing the risk of VITT at its early stage. Specifically, we used the method to demonstrate image-based single-cell profiling and temporal monitoring of platelet aggregates (thrombosis risk) in the blood of healthy human subjects before and after receiving multiple Pfizer-BioNTech vaccine shots in a time span of nearly a year. Results show that the average concentration of platelet aggregates in healthy subjects after the first, second, and third vaccinations is comparable to that before the first vaccination and is lower than that in COVID-19 patients, indicating that the vaccine-induced platelet activity and potential risk of VITT in the healthy subjects were found to be low.